Recently, proposals for an instrument indicating the total distance traveled by an automobile concern an electronic odometer employing non-volatile memories as a substitute for a mechanical instrument which indicates the total distance traveled by a vehicle.
An electronic odometer is powered by the battery of the automobile, and digitally displays the total distance traveled, which is integrated by a counter. According to this device, data on distance traveled is written into a plurality of non-volatile memories every 100 meters, and then that value is read out and indicated, in order to prevent loss of the data on distance traveled as integrated by the counter if the voltage should drop such as when the battery is changed or a wire breaks. At the moment, however, there is a limit on the number of times data can be written into non-volatile memories. With this method, therefore, the odometer can be used for only about ten thousand kilometers, although the automobile can, in practice, travel a total distance of more than a hundred thousand kilometers. One method of solving this problem is disclosed in Japanese Patent Laid-Open No. 95813/1980 laid open in Japan on July 21, 1980, and entitled "Odometer", and involves a method of reducing the number of times data is written into the non-volatile memories. Japanese Patent Laid-Open No. 93209/1977 laid open in Japan on June 10, 1982 and entitled "Odometer for Automobile" discloses a method in which the distance traveled is determined based upon a majority rule, by writing the distance traveled into a plurality of non-volatile memories and then reading the written data out of the non-volatile memories. In either case, data is written repeatedly into the same locations of non-volatile memories, data transfer is destroyed during the writing period (about 300 ms), so that incorrect data could be input to the non-volatile memories unless a large capacitor is provided to back up the power source or unless a large-capacity storage battery is provided.